1. Field of the Invention
The present invention relates to a semiconductor device using a high-resistance element, and more specifically, to a semiconductor device in which a high-resistance element is used in an analog circuit.
2. Description of the Background Art
Conventionally, a high-resistance element has been used in an analog circuit of many LSI (Large Scale Integration) devices such as memory LSI devices. In the analog circuit, the characteristics of the high-resistance element significantly effect the operation of the circuit, often determining the characteristics of the LSI device itself.
As a representative example, in a reference potential Vref generating circuit, fluctuation of the resistance value of a high-resistance element may even changes the reference potential generated by the circuit.
In the following, portions of a conventional semiconductor device where a high-resistance element is used will be described referring to top views or cross-sectional views of FIGS. 16 to 18.
As shown in FIGS. 16 to 18, a conventional semiconductor device using a high-resistance element includes a well 101 doped with N type impurity, an element separating insulation film 102 formed from a main surface of the well 101 to the prescribed depth, a high-resistance element 103 consisting of a diffusion layer of P type impurity and surrounded by the element separating insulation film 102, contact plugs 104a and 104b connected to the high-resistance element 103, an interlayer insulation film 105 in which the contact plugs 104a and 104b are buried, interconnection layer 106a formed on the interlayer insulation film 105 and connected to the contact plug 104a, an interconnection layer 106b connected to the contact plug 104b, an interlayer insulation film 107 formed to cover the interconnection layers 104a and 104b respectively, and an upper interconnection layer 108 formed on the interlayer insulation film 107.
As shown in FIGS. 16 to 18, resistance value of the high resistance element 103 is determined by resistance value per unit area and dimension of the P type impurity diffusion layer. For example, the resistance value R of the high-resistance element 103 may be expressed as follows:
R=Rpxc3x97L/W
where Rp is resistance value per unit area of the P type impurity diffusion layer, L is the length of the P type impurity diffusion layer, and W is the width of the P type impurity diffusion layer. When R is constant, the resistance value of the high-resistance element 103 fluctuates corresponding to the degree of effect, as indicated by arrows 125 in FIG. 18, from changes in potential of the upper interconnection layer 108.
More specifically, the structure shown in FIG. 18 including the upper interconnection layer 108, interlayer insulation films 105, 107, and the high-resistance element 103 is similar to the structure of an MOS transistor including a gate electrode, a gate insulation film and a channel region. As such, fluctuation of the potential of upper interconnection layer 108 between xe2x80x9cHxe2x80x9d and xe2x80x9cLxe2x80x9d changes state of charges distributed in high-resistance element 103 configured with P type impurity diffusion layer. As a result, the resistance value of high-resistance element 103 changes, which in turn changes the amount of current flowing through high-resistance element 103.
One possible technique to solve the above mentioned problem is a structure in which interconnection layer 106b is formed to substantially cover the region above high-resistance element 103, as shown in FIGS. 19 to 21. According to the structure, even when the amount of current flowing through upper interconnection layer 108 changes, the portion of interconnection layer 106b formed above high-resistance element 103 suppresses electrical effect of the upper interconnection layer 108 as indicated by arrows 125. Thus, the resistance value of high-resistance element 103 is prevented from fluctuation.
Specifically, in a semiconductor device shown in FIGS. 19 to 21, interconnection layer 106b extends to the region between the contact plug 104a and contact plug 104b directly above the high-resistance element 103. Thus, the interconnection layer 106b shields high-resistance element 103 from the effect from changes in potential of the upper interconnection layer 108. It is accomplished by the fact that the potential of the upper interconnection layer 106b does not fluctuate as that of the upper interconnection layer 108 and is always identical to that of the contact plug 104b connected to high-resistance element 103.
In the above mentioned semiconductor device, however, the extending length of interconnection layer 106a and that of interconnection layer 106b are different. In other words, the interconnection layer 106a and the interconnection layer 106b are asymmetric to high-resistant element 103. Thus, electric effects on the resistance value of high-resistance element 103 caused by interconnection layer 106a and that caused by interconnection layer 106b are different. Therefore, depending on whether an interconnection layer electrically connected to the high potential electrode side is connected to interconnection layers 106a or 106b, the resistance value of high-resistance element 103, and in effect the amount of the current flowing therethrough, will be different. As a result, on designing a semiconductor device, there has been a limitation on degree of freedom in the layout of an interconnection layer connected to the high potential electrode side, which is to be electrically connected to high-resistance element 103 via interconnection layers 106a and 106b. 
The object of the present invention is to provide a semiconductor device with improved degree of freedom on designing layout of an interconnection layer electrically connected to a high-resistance element, or an interconnection connected to conductive unit, via contact plugs.
A semiconductor device using a resistance element according to a first aspect of the present invention includes a semiconductor substrate, a resistance element formed above or within the semiconductor substrate, an interlayer insulation film formed on the resistance element, a first contact hole penetrating vertically the interlayer insulation film and connected to the resistance element, a second contact hole penetrating vertically the interlayer insulation film and connected to the resistance element, a first interconnection layer formed on the interlayer insulation film and connected to the first contact hole, and a second interconnection layer formed on the interlayer insulation film and connected to the second contact hole. And above the region between the first and second contact holes, the first and the second interconnection layers are formed symmetrical to the prescribed plane perpendicular to the semiconductor substrate, or formed into layers of identical thickness and at the same height and in point symmetry on a prescribed plane parallel to the semiconductor substrate.
According to the above structure, above the region between the first and second contact holes, respective electrical effects to the resistance value of the resistance element by the first and the second interconnection layers become equivalent, thus the degree of freedom is improved on designing interconnection layers respectively connected to the first and second interconnection layers.
A semiconductor device using a resistance element according to a second aspect of the present invention includes a semiconductor substrate, a resistance element formed above the semiconductor substrate, an interlayer insulation film formed under the resistance element, a first contact hole penetrating vertically the interlayer insulation film and connected to the resistance element, a second contact hole penetrating vertically the interlayer insulation film and connected to the resistance element, a first conductive unit formed under the interlayer insulation film and connected to the first contact hole, and a second conductive unit formed under the interlayer insulation film and connected to the second contact hole. And under the region between the first and the second contact holes the first and the second conductive units are formed symmetrical to the prescribed plane perpendicular to the semiconductor substrate, or formed into layers of identical thickness and at the same height and in point symmetry on a prescribed plane parallel to the semiconductor substrate.
According to the above structure, under the region between the first and second contact holes, respective electrical effects to the resistance value of the resistance element by the first and the second conductive units become equivalent, thus the degree of freedom is improved on designing interconnection layers respectively connected to the first and second conductive units.
A semiconductor device using a resistance element according to a third aspect of the present invention includes a semiconductor substrate, a resistance element formed above the semiconductor substrate, a first interlayer insulation film formed under the resistance element, a second interlayer insulation film formed on the resistance element, a first conductive unit formed under the first interlayer insulation film, a second conductive unit formed under the first interlayer insulation film and not being identical to the first conductive unit, a third conductive unit formed on the second interlayer insulation film, a fourth conductive unit formed on the second interlayer insulation film and not being identical to the third conductive unit, a first contact hole penetrating vertically the first and the second interlayer insulation films and connected to the first and the third conductive units, a second contact hole penetrating vertically the first and the second interlayer insulation films and connected to the second and the fourth conductive units, a third contact hole penetrating vertically the second interlayer insulation film and connected to the resistance element and the third conductive unit, and a fourth contact hole penetrating vertically the second interlayer insulation film and connected to the resistance element and the fourth conductive unit. And under the region between the first and the second contact holes, the first and the second conductive units are formed symmetrical to the prescribed plane perpendicular to the semiconductor substrate, or formed into layers of identical thickness and at the same height and in point symmetry on a prescribed plane parallel to the semiconductor substrate, and above the region between the third and the fourth contact holes, the third and the fourth conductive units are formed symmetrical to the prescribed plane perpendicular to the semiconductor substrate, or formed into layers of identical thickness and at the same height and in point symmetry on a prescribed plane parallel to the semiconductor substrate.
According to the above structure, under the region between the first and second contact holes, respective electrical effects to the resistance value of the resistance element by the first and the second conductive units become equivalent, thus the degree of freedom is improved on designing interconnection layers respectively connected to the first and second conductive units. Similarly, under the region between the third and the fourth contact holes, respective electrical effects to the resistance value of the resistance element by the third and the fourth conductive units become equivalent, thus the degree of freedom is improved on designing interconnection layers respectively connected to the third and the fourth conductive units.